Transparent ceramics have recently acquired a high level of interest. In particular, electro-optic ceramics, where the property of the ceramic material can be tuned by an externally applied electric field, have found extensive applications in devices such as optical fibres for guided light wave transmission, optical switches, variable optical attenuators, polarization controllers, tuneable optical filters, optical shutters and night vision goggles.
Electro-optic ceramics can be made of a single crystal or be polycrystalline. Single-crystal electro-optic ceramics may be largely defect-free and display better electro-optic performance. However, the processing of a single crystal is extremely expensive. In contrast, polycrystalline materials are low cost and display good electro-optic effects and ceramic ruggedness. However, the optical transparency of polycrystalline materials is limited by light scattering caused by their microstructural features. Since the extent of light scattering depends on the wavelength of the incident radiation or light, polycrystalline electro-optic ceramics that have scattering centres on a similar spatial scale to the wavelengths of visible light, that is, on the order of hundreds of nanometers, display the most light scattering and therefore the least transparency.
Among the polycrystalline electro-optic ceramics, the most widely used are based on the PbTiO3—PbZrO3 (PZT) solid solution. Although PZT ceramics show good electro-optical properties, good transparency, ceramic ruggedness and a low cost of production, most of the ceramic materials within the PZT family are composed of about 60 wt % of lead, which raises ecological concerns. Some countries have legislated to replace PZT with lead-free ceramics, since lead is a toxic element that may affect the human health and the environment. Due to this, recent research on electro-optic ceramics has focused on the development of lead-free electro-optic ceramics.
Among the lead free ceramics developed so far, the K0.5Na0.5NbO3 (KNN) solid solution has received particular interest as it has displayed the most promising results. However, KNN ceramic materials suffer major drawbacks in its processing steps. For example, the alkaline elements undergo sublimation at the high temperatures required to achieve adequate densification, changing the initial stoichiometry considerably. Since the properties of the materials are highly sensitive to stoichiometry, it is difficult to control the fabrication process to achieve precise compositions of the materials.
Densification is a crucial step in obtaining highly transparent and efficient electro-optical ceramic materials, hence there have been many attempts to improve this property. One way to improve densification is to reduce the particle size of the synthesised powders. Conventional solid-state ceramic synthesis methods do not achieve considerable reduction of particle size, hence KNN-based compositions have been obtained through various soft chemistry routes. However, these methods are often costly, time-consuming, require strict control of the reaction conditions and are generally inefficient.
There is therefore a need to provide an electro-optic material that overcomes, or at least ameliorates, one or more of the disadvantages described above. Further, there is a need to provide an electro-optical device that comprises the electro-optical material, that is ecologically safe, has high transparency, displays high and fast electro-optical effects, a wide window of transparency, is operable at room temperature and is cost-efficient. There is also a need to provide a method for fabricating such an electro-optic material and device that overcomes, or at least ameliorates, one or more of the disadvantages described above.